1. Field of the Invention
The present invention relates to an antenna device and a radio apparatus, and in particular to a tunable antenna device and a radio apparatus including the tunable antenna device.
2. Description of the Related Art
Radio apparatus such as mobile phones are now being used more widely and in a broader range of applications. Some kinds of mobile phones, e.g., may receive digital terrestrial television (TV) broadcasting (DTTB service for mobile phones, so called “1seg” in Japan). While being required to be small sized of less thickness, such a radio apparatus, e.g., a mobile phone, needs to cope with more limited component mounting space as being used for multiple applications.
In above circumstances, a radio apparatus needs an antenna device simultaneously satisfying requirements of a smaller size and a broader frequency band (e.g., 470-770 megahertz (MHz) for receiving DTTB) which are likely to conflict. Possible solutions to the above need are disclosed in Japanese Patent Publication of Unexamined Applications (Kokai), No. 2006-140662, No. 2006-270916, No. 2006-319477 and No. 2006-345042.
More specifically, an antenna disclosed in JP 2006-140662 is formed by a main portion composed of dielectric or magnetic material and two radiation conductors wound around the main portion. The radiation conductors are connected in series through a switch. One of the radiation conductors is on a feeder side and is loaded with variable capacitors on every other turn. The antenna of JP 2006-140662 may change a resonant frequency between a VHF band and a UHF band.
An antenna disclosed in JP 2006-270916 is formed by a stick-like shaped piece of dielectric or magnetic material on which a linear conductor pattern is formed. An inductor portion and a frequency adjusting portion are arranged between an end of the conductor pattern and a grounded conductor, and the end is configured to be fed. The antenna of JP 2006-270916 may be tuned in the 470-770 MHz frequency band by adjustment of a variable capacitor included in the frequency adjusting portion.
An antenna disclosed in JP 2006-319477 includes a radiation element configured to cover a frequency band for mobile phones and the frequency band for DTTB. The radiation element is connected to a feeding point through an inductive element, a tuning circuit and a filter. The antenna of JP 2006-319477 includes a parasitic element arranged close and coupled to the radiation element, and connected to the above feeding point through another filter. The antenna of JP 2006-319477 configured as described above may be used as a Tunable antenna not only for a mobile phone but also for receiving DTTB.
An antenna disclosed in JP 2006-345042 is formed by two transmission lines having a common feeder end and each of which is shorter than a quarter wavelength of a used frequency. Another end of one of the transmission lines is grounded, and another end of another one of the transmission lines is grounded through a variable capacitor element. The antenna of JP 2006-345042 has a resonant frequency that may be controlled by adjustment of the variable capacitor element.
The antennas described above may be tuned to a frequency in the frequency band for DTTB, or may be switched over between use for receiving DTTB and use for mobile communication.
As technologies of digital image recording such as an application of recording digital TV broadcasting, meanwhile, has made a progress, a small-sized radio apparatus such as a mobile phone may be equipped with a function of recording digital images (moving pictures, in particular). It is generally true that a fixed TV set has two tuners for receiving digital TV broadcasting to be simultaneously used, one of which is for watching a program on a channel and another one of which is for recording a competing program on another channel. On this occasion, each of the tuners may have and feed an own antenna.
A small-sized radio apparatus such as a mobile phone may similarly be equipped with a function of recording a competing program. It is difficult for such a small-sized radio apparatus, however, to have plural antennas each of which is individually fed, as mounting space is strictly limited in comparison with the fixed TV set. Thus, such a radio apparatus may need an antenna configured to be not only tuned to a frequency in a broad frequency band but also individually tuned to each of plural frequencies. It is obvious that none of the antennas of JP 2006-140662 and so on described above may meet the above need.